1. Field of the Invention
The present invention relates to a semiconductor laser device using double heterostructure comprised of semiconductive material of composition In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1).
2. Description of the Background Art
Lately, much attention has been paid for a visible light semiconductor laser device using double heterostructure comprised of semiconductive material of composition In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) manufactured as metalorganic materials grown on GaAs substrate by metalorganic chemical vapor deposition (referred hereafter as MOCVD) or by molecular beam epitaxy (referred hereafter as MBE). In particular, crystalline InGaAlP among the group III-V composite semiconductive materials is considered to be important in constructing a short wavelength semiconductor laser device. There has been reports of such devices with over 1,000 hours lifetime.
Now, a threshold current for lasing of a semiconductor laser device should preferably be low, so as to reduce amounts of current in operational state as well as to improve lifetime characteristics of the device. This threshold current is determined by an amount of carriers leaking through an active layer and, in particular, it is considered that preventing the leakage of electrons of small effective mass into a p-type cladding layer has significant effect.
In this regard, a conventional laser device using Ga.sub.1-r Al.sub.r As (0.ltoreq.r.ltoreq.1) has little problem in reducing the threshold current, since in such a conventional laser device it is possible to make a conduction band discontinuity between an active layer and a p-type cladding layer sufficiently large which can contribute to the prevention of the leakage of electrons from the active layer into the p-type cladding layer.
However, since In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) has a smaller conduction band discontinuity than Ga.sub.1-r Al.sub.r As (0.ltoreq.r.ltoreq.1), it has been difficult for a semiconductor laser device using In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) to achieve a sufficient prevention of the leakage of electrons from the active layer into the p-type cladding layer, or in other words, an effective confinement of carriers.
Thus, conventionally a semiconductor laser device using In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) has been associated with a relatively high threshold current and various disadvantages arising from this such as those mentioned above.
In addition, a semiconductor laser device using In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) has other difficulties concerning a threshold current density and a characteristic temperature.
Namely, the band gap energy of a semiconductor laser device using In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) grown on GaAs substrate can be varied between approximately 1.8 eV and 2. 35 eV by changing the amount of Ga and Al, i.e., x and y in the expression. This range of the band gap energy corresponds to the lasing wavelength of approximately 0.68 .mu.m to 0.56 .mu.m. Actual compositions most commonly employed for such a semiconductor laser device are the active layer of Ga.sub.0.5 In.sub.0.5 P and the p-type cladding layer of (Al.sub.z Ga.sub.1-z).sub.0.5 In.sub.0.5 P (0.ltoreq.z.ltoreq.1). The value of z is typically chosen to be between 0.4 and 0.5.
However, photoluminescence (PL) peak wavelengths at room temperature (25.degree. C.) of such a semiconductor laser device measured as a function of z, which are shown in FIG. 1 where the vertical axis is wavelength .gamma. and horizontal axis is z, shows that the expression: EQU .lambda.=-172z+662 (nm)
holds for z&lt;0.7. From this information, the band gap .DELTA.Eg between the active layer and the p-type cladding layer can be derived to be .DELTA.Eg=0.215 eV for z=0.4, and .DELTA.Eg=0.28 eV for z=0.5, both of which failing to satisfy .DELTA.Eg.gtoreq.0.3 eV which is considered to be a necessary condition for a semiconductor device to be operative with low threshold current, at a high temperature. In fact, a sample semiconductor laser device of whole electrode type possessed very large threshold current density of 3 kA/cm.sup.2 for z=0.4, and relatively large threshold current density of 1.6-2 kA/cm.sup.2 for z=0.5. Moreover, the characteristic temperature was small for this sample semiconductor laser device so that emission at high temperature was not obtainable.
Thus, conventionally it has been very difficult, if not impossible, to make a semiconductor laser device using In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) having the band gap .DELTA.Eg.gtoreq.0.3 eV, so that the threshold current density has been large and the characteristic temperature has been small.
Furthermore, use of MOCVD for growing In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) give rises to further problems related to doping.
Namely, MOCVD has conventionally been carried out with selenium (Se) obtained from hydrogen selenide (H.sub.2 Se) as n-type dopant. Yet, there has been scarcely any study on controllability of Se-doping applied to In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1). With this respect, the doping efficiencies of Se-doping from H.sub.2 Se in MOCVD for growing on GaAs substrate, In.sub.x Ga.sub.y Al.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1), and GaAs are shown in FIG. 2. In addition, substrate temperature dependences of the doping efficiencies for InGaP and InAlP are shown in FIGS. 3 and 4, respectively.
From FIGS. 2, 3, and 4, the following problems of growing of Se-doped InGaAlP by MOCVD can be deduced:
(i) The doping efficiencies depend heavily on composition, which makes uniform doping difficult, especially at boundaries of different compositions.
(ii) When InGaAlP is grown on GaAs substrate such that the density of Se in GaAs becomes about 1.times.10.sup.18 cm.sup.-3, due to diffusions and other effects the density of Se in InGaAlP nearby GaAs substrate becomes almost 1.times.10.sup.19 cm.sup.-3 as a doping efficiency of InGaAlP is greater than that of GaAs by order of one, which greatly lessens the lifetime of the device.
(iii) The doping efficiencies also depends heavily on the substrate temperature and, in particular, considerable variation in the carrier density in InAlP is caused by a slight variation in the substrate temperature, which makes stable doping difficult.